I think what Klassic was trying to say (it looks like she has a tough auto-correct) was that you should work on loosening up your upper body and reaching more forward into the jump.
So you can start your rides by stretching your arms, stretch them out to either side (like and airplane) make circles with your arms. Roll your shoulders in circles backwards to help open up your diaphragm and relax your upper body.
Then when you jump and go into your half seat you want to stretch your arms up a little less than half way up the horse's neck and stand well up out of the saddle. You should practice this regularly at the trot and canter to work on your balance before jumping like this. This will help your seat a great deal because it will help align your lower leg and keep your balance. You should be able to comfortably reach up and rest your hands on the horse's neck, standing up off the horse's back without actually resting weight on the horse's neck - this is tougher than it sounds ;)
I think that will help a great deal to allow the horse a little more room to jump.
While I love that your hands are so still and quiet, but they should be a bit more forward in general, and much more forward over the jump :)
So work on loosening up your upper half, stretching up into a full half seat when jumping and standing up well off the horse's back during the jump.
This diagram isn't perfect, but you can get the idea, when actually jumping I'd like to see the person even more forward than this: